They have found the Mass Air Flow sensor to be giving a faulty reading.

The problem with the car was sometimes when started up it would run under powered for a few miles then cure itself. Sometimes it would run fine from startup, others it would not. It never became underpowered mid journey while the engine was running though it would sometimes if the engine was stopped then started again.

Does this sound like a faulty Air flow sensor or is it more likely to be something else? I don't want to shell out £180 to find out it's not the problem.

What I find strange is how the problem only ever seems to occur following startup, not mid journey.

Also how come there were no error codes stored in the management system. I've been told it only logs them if the sensor reads badly for a certain period of time or if the sensor value is over a certain threshold.

I believe the car performance is because the management system is going into "limp home mode" so why is the engine management light not coming on and a faultcode logged?

The MAS is situated on the air intake you can easily take it out and you will see what a simple affair it is.You can measure the resistance easily if you know what it should be and compare .I would also think that there are many servicable parts available in the scrap yards,infact a MAS from any Vectra will work regardless of the engine size.It is common practice to fit a MAS with the capability to accept much higher air flows when you change to something like a K&N filter or such like,the more cold air you can get into the engine the better its performance.

So with that said it sounds strange that my intermittent fault tends to occur just after the car has been started and not mid journey. Surely a faulty sensor would screw up my engine running or force the car to a preset running value at any stage during a journey. With the running problems I have described would you put them down to a faulty MAS or do you think there is another issue here?

I have a Cavalier with 2 liter Ecotec engine, on several occaisions my car has gone into limp home mode due to faulty sensors usually cam and cranck. Limp home mode only seems to prevent engine running over 4000rpm, performance below that does seem very different from norm.
I would suggest you check out the "Cavweb" website which is full of lots of info on EMU sensors and some pretty knowledgeable people.

If the garage say it will fix the problem, then go ahead. If it turns out not to be, then don't pay them. That's what garages are for, to solve problems, not just bolt on parts until they find the right one. It could be that they don't have a code reader, so they are using the old fashioned (and probably better) method of checking the spec of each component.

If think some MAF sensors use a hot wire to measure flow, which could point to why it plays up when cold.

The reason that a fault is not logged or the MIL light stays off is because although incorrect, the reading from the sensor is plausible. The Tech instrument GM (ab)use will actually give 'live' data from the sensor with the engine running which can be compared with known data, and it is probably this that is suggesting to the garage that a replacement is necc. Try cleaning the unit first with an electrical contact cleaning spray sprayed onto the hotwires.
HTH
Andrew.

Correct me if i am wrong but the gadget you refer to supplies the engine with air, in the old days you could twist the air filter to intake air from the manifold , this was the cold weather position, a further development from this was a temperature sensitive flap which did the same thing but automatically selected the correct air temperature required dependant i think on engine temperature, this aided cold running.
When the car is up to temperature air will normally be from other than the manifold.
Newer systems work in a similar way but with ECU control. the feedback position signal is probably giving a false position reading (say engine warm position) so the ECU is injecting a fuel air ratio for that reading , this will cause rough running till up to temperature.

I wouldn't say the engine was running particularly rough it just runs very slowly. It starts perfectly, idles just right and responds to the throttle but just doesn't accelerate quickly.

If the Mass air flow meter gives an incorrect value on startup what happens?

Does the engine go into "safety mode" and therefore the sluggishness is due to that or will it use the value and give an incorrect fuel/air mix and run rough? It feels to me like the engine is running fine, just underpowered which suggests the former.

If the answer is that the engine is put in safety mode is there a way of over-riding it? I ask because sometimes it will run sluggish from start on a warm engine but if the car is stopped and started again it sometimes cures itself.

One of the functions incorporated into the system is a 'burn off' period when the engine is switched off. This momentarily heats the hotwires up to clean off any deposits covering the wires. Could be a fault here which occasionally works only when you switch off and restart.